1. Field of the Invention
The present invention relates to a CDMA communication device and CDMA communication method.
2. Description of the Related Art
In CDMA (Code Division Multiple Access), which employs spread spectrum technology in radio transmission, the channel symbol rate and the spread factor (hereinbelow, referred to as “SF value”) are inversely proportional to each other in an environment in which chip rates are identical.
In a CDMA base station, changing the size of the SF value enables both changes to various transfer rates as well as the multiplexing and transmission of the user data of a plurality of users.
The maximum number of users for which user data can be multiplexed in a CDMA base station is determined by the Signal-to-Interference Rate (hereinbelow, referred to as “SIR value”) of the user data when the user data that are transmitted from the CDMA base station are despread and decoded at users' mobile devices.
Thus, in a CDMA base station, multiplexing user data to the minimum extent of SIR value that can still guarantee the quality of various communication services allows the communication services to be offered to more users.
The composition of the transmission components of a CDMA base station of the prior art is shown in FIG. 1.
Referring now to FIG. 1, a CDMA base station of this example of the prior art includes: spreading unit 100 for spreading and multiplexing user data #1-#n, control device 600 for controlling spreading unit 100, radio transmitters 3001-300m that are provided correspondingly to each of m frequencies f1-fm for modulating the user data that have undergone spreading at the corresponding frequency, combiner 400 for combining user data that have undergone modulation, and antenna 500 for the radio transmission of user data that have undergone combining. Antenna 500 can share the m frequencies f1-fm.
Spreading unit 100 includes: spreader 110, adder selector circuit 120, and multiple-input adders 1301-130m that are provided correspondingly to each of m frequencies f1-fm.
Spreader 110 receives user data #1-#n as input and spreads the received user data #1-#n. Spreader 110 is connected to control device 600, and the SF value when spreading user data #1-#n is set by means of control device 600.
Adder selector circuit 120 is connected to control device 600 and distributes user data, which have undergone spreading, to multiple-input adders 1301-130m each corresponding to one of the frequencies that have been set by control device 600. Control device 600 controls the assignment of each user's frequency to one of frequencies f1-fm, i.e., controls the assignment and transmission of user data #1-#n for each user to any of radio transmitters 3001-300m at frequencies f1-fm. As the control method, the working frequency may be determined by, for example, round-robin scheduling. Technology for assigning a carrier (frequency) to each user in this way is disclosed in, for example, Japanese Patent Laid-Open Publication No. 2003-046474.
Multiple-input adders 1301-130m are each connected to respective radio transmitters 3001-300m, multiplex user data that have undergone spreading, and supply the multiplexed user data to the radio transmitters 1301-130m to which the respective multiple-input adder are connected.
The operation of the CDMA base station in this example of the prior art is next explained.
Spreader 110 first spreads user data #1 -#n by the SF value that has been set by means of control device 600 and supplies the result to adder selector circuit 120.
Adder selector circuit 120 next selects one of multiple-input adders 1301-130m each corresponding to the respective frequencies that have been set by means of control device 600, and supplies the user data that have undergone spreading and that have been supplied as input from spreader 110 to the selected one of multiple-input adders 1301-130m.
Each of multiple-input adders 1301-130m next multiplexes the user data that have undergone spreading and that have been supplied as input from adder selector circuit 120, and then supplies the user data that have undergone multiplexing to one of radio transmitters 3001-300m to which the multiple-input adder is connected.
Radio transmitters 3001-300m modulate the user data, which have undergone multiplexing and have been supplied as input from multiple-input adders 1301-130m, with respective frequencies f1-fm.
The user data that have undergone modulation by radio transmitters 3001-300m are next combined by combiner 400 and radio-transmitted from antenna 500.
As described in the foregoing explanation, when assigning the user data of each user to a radio transmitter in the CDMA base station of the prior art that is shown in FIG. 1, the assignment is realized in accordance with round-robin scheduling without consideration of the SF value.
If the user data of a new user are assigned to a radio transmitter that already holds the user data of other users, user data having a large SF value can be accommodated in the radio transmitter. However, if the user data has a small SF value, the possibility arises that an SIR value that can guarantee communication service will not be secured and these user data will not be accommodated in the radio transmitter. The occurrence of this problem results in both the occurrence of call loss and a deterioration of the usability of the frequencies.
The number of radio transmitters can be increased as a means of dealing with this problem, but this is not a true solution because, if attention is placed on any particular radio transmitter, the possibility remains that the same problem will occur. In addition, there is the additional problem that increasing the number of radio transmitters results in an increase in the circuit scale.